1. Field of the Disclosure
The disclosure relates generally to a control device for driving assistance for a vehicle, such as an automobile. More specifically, the disclosure relates to an automatic deceleration control device for a vehicle, the automatic deceleration control device being configured to automatically reduce the vehicle speed while the vehicle is traveling.
2. Description of Related Art
In a technical field of driving assistance control for a vehicle, such as an automobile, there is a method of controlling traveling of the vehicle with reference to the conditions and environments surrounding the vehicle in motion, based on the road map information acquired from a global positioning system (GPS) device or the information acquired by an in-vehicle video camera or an in-vehicle radar sensor. For example, according to Japanese Unexamined Patent Application Publication No. 2006-193095 (JP 2006-193095 A), the road-type information about a road on which a vehicle is traveling is acquired from a global positioning system (GPS) navigation device, and the vehicle speed is controlled to be adjusted to a vehicle speed that is preset based on the type of the road on which the vehicle is traveling. In this case, the vehicle speed desired by a driver and the speed limit for the road are compared with each other, and the lower one of these speeds is employed as the vehicle speed that is preset based on the type of the road on which the vehicle is traveling. According to Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2015-516623 (JP 2015-516623 A), vehicle control is performed with reference to, for example, the vehicle speeds of other vehicles around a host vehicle.
When decelerating a vehicle, such as an automobile, a driver of the vehicle typically performs a vehicle decelerating operation such that the vehicle decelerates at an appropriate degree (deceleration), based on the type of a road on which the vehicle is traveling. Note that, roads are classified into a plurality of road types depending on the kinds of roads, such as main roads, junction points, merging points, exit points, bridges, tunnels, and climbing lanes, and variations in, for example, the number of lanes, road width, and curvature, in highways or ordinary roads. More specifically, for example, when the speed limit is reduced while the vehicle is traveling on a main road of a highway, the driver applies a brake to the vehicle so as to decelerate the vehicle at a relatively low deceleration, so that the traffic flow around the vehicle is not disturbed. On the other hand, when the vehicle is traveling on a branch road leading to an exit of an interchange, the driver applies a brake to the vehicle so as to decelerate the vehicle at a high deceleration. That is, it can be said that, in many cases, the driver performs a vehicle decelerating operation based on the type of a road on which the vehicle is currently traveling, so that occupants of nearby vehicles and the host vehicle do not feel uncomfortable. If the deceleration of a vehicle (host vehicle) is actually excessively high in the traffic flow around the vehicle, the subsequent vehicle is decelerated or steered in response to relatively sharp reduction in the vehicle speed of the preceding vehicle (host vehicle), so that the traffic flow may be disturbed on the road on which the vehicle is traveling. The braking operation performed by the driver may vary depending on the type of a road on which the vehicle is traveling. For example, when the driver determines the type of a road on which the vehicle is traveling and then applies a brake to the vehicle such that the vehicle is decelerated at a relatively low deceleration, the driver just cancels depression of an accelerator pedal without depressing a foot brake pedal. Thus, only engine braking is used to apply a brake to the vehicle. Only after it is determined that a relatively high deceleration is required, the foot brake pedal is depressed. Desirably, these modes of operations that are performed by the driver in decelerating the vehicle are achieved also in a case where the vehicle speed is automatically controlled in the vehicle driving assistance control such that the driver and passengers do not feel uncomfortable.